PHAZE II sounding rocket observations of VLF waves and plasma density measurements made with a HF wave instrument rather than a fixed bias Langmuir probe show conclusively that the observed lower hybrid solitary structures (LHSS) are associated with density fluctuations. LHSS occur on the positive and negative gradients of plasma density fluctuations of 25--50%. These findings lend support to theories of ambient lower hybrid waves exciting resonances created by pre-existing density gradients rather than wave collapse or refraction mechanisms; the issue of how the gradients are created is still unresolved. ¿ 1998 American Geophysical Union